Many electronic devices use consumer-replaceable batteries to power the device. Consumer-replaceable batteries are sold in standard sizes and voltages, with 1.5 volt, AA-sized zinc-manganese dioxide (i.e., alkaline) batteries being the most common primary battery. Other primary and secondary battery types found in the market today may take a variety of forms, from cylindrical (AA, AAA, C, D, etc.) to button or coin sized cells. The battery chemistry for such batteries is equally broad, including nickel-metal hydride, lithium-iron disulfide, lithium-manganese dioxide, zinc-air and carbon-zinc electrochemical systems, with the voltage for each dictated by the electrochemical reaction inherent thereto. While most consumer-replaceable batteries output nominal, direct voltages of 1.5 or 3.0 volts, the foregoing demonstrates that consumer-replaceable batteries encompass a wide range of possibilities. It should be noted that the International Electrotechnical Commission and the American National Standards Institute publish specifications for such batteries.
Although devices consumer-replaceable batteries cover a wide range of different applications, flashlights—and particularly flashlights utilizing at least one light emitting diode (“LED”)—are perhaps the most ubiquitous. Device manufacturers tend to prefer consumer-replaceable batteries in flashlights for their low cost, convenience, portability and familiarity.
Prior to the development of LEDs, flashlights used incandescent light bulbs. These bulbs employ current to heat a wire filament to produce incandescent light. However, most batteries—and particularly alkaline batteries that are most commonly used in such devices—experience a steady decline in output voltage over the period of time during which the battery is discharged. As a result, incandescent flashlights often experienced a corresponding degradation in light output as the batteries were discharged. For purposes of this disclosure, the relationship between battery degradation and lighting performance can be characterized as passive, insofar as no circuitry or hardware is provided to address this issue.
As LEDs began to replace incandescent bulbs in flashlights, a separate series of issues arose. First, LED output (i.e., the quality of the light as perceived by an observer) depends, in part, on the current fed to the LED and the temperature of the LED itself. If the battery current is not tuned to the LED requirements, excess heat will be generated according Joule's First Law and, as a consequence, the LED performance will be impaired. In a similar manner, the declining output voltage of batteries during discharge, and particularly alkaline batteries (which exhibit a more pronounced decline in comparison to the chemistry of other batteries, such as lithium-iron disulfide), necessarily and passively impairs the LED output.
In response to these issues, LED flashlight manufacturers employed a number of approaches. The most prevalent approach was to employ circuitry or hardware to maintain a constant current for the LED. FIG. 1A generally illustrates how the battery current, Ibat, and the LED current, LED, would behave over time. In practice, this approach is active insofar as it requires hardware to be interposed between the battery and the LED in order to make the necessary adjustments. This approach effectively established a feedback loop between the LED and the driver circuit (e.g., an integrated chip) in order to insure the current was maintained at a constant level.
U.S. Pat. No. 8,299,726 discloses a power supply for an LED device that sense current through an LED and utilizes a switch-mode regulator to provide a substantially constant current output to the LED by adjusting the switch-mode regulator as a function of the current sensed.
U.S. Pat. No. 6,586,890 discloses a system for powering an LED including an oscillator that supplies output power to the LED only when the oscillator signal is in a particular condition.
U.S. Pat. No. 9,055,634 describes an LED light source energized with an effective power in accordance with a targeted illuminating resistance. Numerous schemes are disclosed for decreasing the voltage and/or supplying a constant current to the LED.
FIG. 1B schematically illustrates a generic output regulation scheme 10 of the prior art. Battery provides a current IB to the input Pin of an integrated circuit 15. The integrated circuit 15 acts as a driver from the LED (or an array of LEDs) by way of its output Pout. A feedback loop FB is established based on the current ILED flowing through the LED so as to allow regulation of output Pout to optimize the performance of the LED.
Another approach to implementing LEDs while minimizing the aforementioned issues was for the flashlight manufacturer simply to design components that were optimized to a particular voltage and discharge profile (i.e., to design the light to a specific battery chemistry). In doing so, it is often assumed that the consumer-replaceable batteries will be primary alkaline cells (i.e., a 1.5 volt zinc-manganese dioxide battery having standardized size/form factor, such as LR6), which possess discharge characteristics that are distinct from other batteries electro/chemical cells. Under this approach, the components (e.g., diodes, heat sinks, etc.) are selected to handle current outputs of an alkaline battery.
When this chemistry-specific type of design is utilized, the use of a different battery has inevitable negative results. For example, when batteries with a higher output voltage and, by operation of Ohm's law, current (e.g., lithium-iron disulfide batteries) are used in such lights, the excess current results in the generation of excess heat. In some cases, the disparity between the battery output and design is sufficient to damage the light and/or to cause its operation and light output to be significantly impaired. While flashlight manufacturers sometimes attempted to overcome this issue by recommending only a particular type of battery, this passive solution may be ineffective because it relies entirely upon a sometimes unsophisticated user to recognize the differences in battery chemistry for cells that otherwise had the same appearance and nominal voltage and act accordingly. In the same manner, schemes to detect battery chemistry can be complex and often too expensive for use in everyday flashlights.
Finally, the implementation of LEDs in lighting devices created another issue: because of an LED's increased sensitivity to current levels, previous schemes for dimming incandescent lights were not feasible. That is, while an incandescent bulb could be dimmed simply by reducing the current so as to reduce the generation of heat and incandescent effect, LED bulbs proved to be too sensitive to current changes to make this approach feasible. Consequently, light manufacturers began to include features such pulse width modulation (PWM) of the current provided to the LED. In these schemes, PWM was designed to essentially deactivate the LED a sufficient number of times within the modulation cycle so as to create a perceptible dimming effect for the user even though the actual light intensity of the LED was not (and cannot be) altered.
For example, United States patent publication no. 2015/0257230 describes an LED driver unit that sense current flowing through an LED array. A dimming control unit then relies on a voltage divider unit in order to dim the LED array.
The inventors are also aware of integrated circuits sold as an LED driver made by Diode, Inc. of Plano, Tex., USA (www.diodes.com/catalog/LED_Drivers_67). These drivers are sold as low voltage DC-DC LED drivers specifically designed for portable lighting applications. Some of these drivers include boost-buck operations to adjust voltage, but the drivers are all generally described as providing high efficiency constant current for high brightness LEDs with both high and low current.
Given the foregoing, there is a need for an LED lighting device that is responsive to the current delivered by the battery so as to prolong run-time rather than maintain specific conditions for LED operation. This need also encompasses a method for driving an LED by establishing a feedback loop between the driver and the battery itself.